Ultrasound is a form of vibrational energy. When it propagates through, and interacts with, a liquid, the energy is attenuated by scattering or absorption. At low ultrasound powers, the energy is absorbed by the liquid in a thermal interaction that causes local heating. At higher powers the interaction becomes increasingly non-linear and both non-thermal mechanical and cavitational mechanisms become significant. The non-thermal mechanical mechanisms can include radiation pressure, acoustic streaming, radiation forces, torques, and near-boundary/bubble hydrodynamic shear forces.
These ultrasonic interactions with a liquid, particularly those involving cavitation, have been exploited for many years in devices that clean or separate materials, accelerate or modify chemical reactions, and kill or lyse cells. Such devices typically utilize sonic horns, or probes, and are designed to optimize the cavitation mechanism at frequencies generally in the range of 20–50 kHz. For comparison, ultrasound devices used in the medical field typically operate at frequencies of 0.8–15 MHz and at lower power densities (<0.5 W/cm2 for diagnostics and ˜0.5–3 W/cm2 for therapy).
Ultrasound offers an attractive cell lysing tool to obtain sufficient amounts of nuclear, cytoplasmic, or other cellular material for commercial use (e.g., proteins), or for analysis and identification (e.g., anthrax or e-coli). Effective and rapid lysing is particularly important for the most refractory microorganisms of concern to public health including protozoan cysts, fungal hyphae, Gram positive bacteria, and spores. In a suspension containing microorganisms, the nature of the ultrasound-suspension interaction is complex and has been shown to depend on at least the power level in the ultrasound, the ultrasound field geometry, and frequency of the ultrasound.
Current ultrasound lysing (and other material processing) devices typically use kHz frequencies with a horn or probe configured to optimize cavitation. For a given frequency, there is a minimum power level necessary to cause cavitation, known as the cavitation threshold. In general, the power necessary to achieve cavitation increases with frequency. Thus, when using MHz frequencies, contrast agents (e.g., microbubbles, microparticles) are often introduced in the liquid to help reduce the cavitation threshold by increasing the mechanical interaction and inducing cavitation-like phenomena. In some MHz applications, it is only with the presence of such contrast agents that cavitation occurs.
Because ultrasonic vibration is rapidly attenuated in passing through long paths in a liquid, it is common to effect cell lysis by applying the cavitating kHz ultrasound in a confined chamber. Current sonic lysing devices typically employ a batch processing approach using static liquid reaction chambers. For example, Belgrader et al (Anal. Chemistry, Vol. 71, No. 19, Oct. 1, 1999) employs a horn-based minisonicator for spore lysis and subsequent polymerase chain reaction analysis. Such devices are prone to erosion of the sonic horn tip and unacceptable heating of the liquid.
A few flow-through devices have been developed, though they still incorporate sonic probes depositing energy in a confined chamber. For example, the flow-through devices disclosed in U.S. Pat. No. 3,715,104 and McIntosh and Hobbs (Proc. of Ultrasounics in Industry, pp 6–8, Oct. 20–21, 1970) agitate a liquid between two closely spaced flat surfaces. Furthermore, T. J. Mason (Ultrasonics, 1992, Vol. 30, No. 3, pp 192–196) discloses other flow-through sonic devices that incorporate transducers symmetrically positioned about the flow path of a liquid.
Most current ultrasound processing devices, however, cannot meet the practical, economical, and operational requirements associated with industrial-scale chemical/physical processing systems, field deployable systems, or continuous biomonitoring systems. This is especially true for systems requiring automation or remote operation. Such systems require rapid, effective, efficient, and near-continuous processing with minimal or no manual steps. As the present invention will illustrate, there is an opportunity to apply non-conventional combinations of ultrasonic power, frequency, and field geometry to address current lysing needs and to improve existing (and develop new) chemical and physical processing methods for materials. In particular, ultrasonic treatment at conditions that avoid conventional cavitation and promote non-thermal mechanical interactions shows great potential.